1. Field of the Invention
The present invention relates to a laser processing method for processing a micro convexo-concave structure on a surface of a glass substrate and an optical diffraction element obtained by the laser processing method, and to a method for manufacturing an optical element including a diffraction grating which is used as a polarized light beam splitter, a coupling grating or the like, a diffraction type optical element which is used for a hologram, or an optical element such as a photonic crystal which is used as a birefringent plate, a light beam scatter plate or the like.
2. Description of Related Art
A glass plate has superior characteristics with respect to flatness, processing accuracy, weather resistance, heat resistance, etc. Therefore, there are already known devices such as a diffraction grating for use in optical communication and so on, or a micro lens for installation in a display device, which are formed by performing micro processing on a surface of a glass substrate.
For treating such micro processed regions on the glass substrate, conventionally, there is generally known a wet etching (chemical etching) method by using an etchant including hydrofluoric acid, etc., or a dry etching (physical etching) method by using a reactive ion etching, etc.
However, there is a problem with the care and the treatment of the etchant in the wet etching, and also a problem in the dry etching in that the facility of a vacuum container is necessitated, thereby requiring a large-scale facility by itself. Additionally, it is not cost-effective because a pattern mask and the like must be formed by a further complicated photolithography technique.
Moreover, an element for dividing wavelength, such as a diffraction grating, etc., which is available commercially at a relatively cheap price, is industrially produced by a method of obtaining an original negative plate by cutting a metal plate of aluminum or the like with a diamond blade (so called xe2x80x9ca ruling enginexe2x80x9d) and transferring upon an epoxy resin on the basis thereof.
In the above-mentioned industrial production method for the diffraction grating, a large-scale facility for the ruling is also necessitated, and as well, the element for dividing wavelength must be transferred onto organic materials for the mass production thereof. The transfer onto the organic materials shows good formability, however, it has a disadvantageous limit with respect to resistance against humidity and excessive temperatures.
On the other hand, it is known that a laser beam has strong energy so that the temperature of an irradiated surface of any arbitrary material increases resulting in ablation or evaporation of the irradiated portion thereof. Therefore, conventionally, a laser beam has been utilized in various processings or machining methods. In particular, the method of using a laser has been adapted to micro processing or machining because the laser beam can be easily focused onto a very fine spot.
Then, in the prior art, various methods for achieving such micro processing are already known, in which a periodic optical intensity distribution of the laser beam is obtained by causing a plurality of laser beams to mutually interfere. The mutually interfering beams are then radiated onto the surface of the material to be processed, such as a metal plate or the like, as disclosed for example, in Japanese Laid-open Patent Nos. Sho 50-42499(1975) and Hei 4-253583(1992), and in Japanese Patent Publication Nos. Hei 7-4675(1995), Hei 7-47232(1995), Hei 7-51400(1995), Hei 7-102470(1995), Hei 8-9794(1996), and Hei 8-25045(1996).
In particular among them, in Japanese Patent Publication No. Hei 8-25045(1996), a wave guide (a thin layer or film) having an index of refraction higher than air and that of the material to be processed is provided on the material to be processed, such as the metal plate or the like, and the laser beam is radiated onto the wave guide, thereby forming micro convexo-concave patterns in the wave guide by interference between the light beams transmitted in the wave guide and the radiated light beam, and providing a rainbow color developing function on the surface of the material to be processed.
Further, in one publication (xe2x80x9cAn Applied Physicsxe2x80x9d, by Masataka Murahara et al., Volume number 52, No. 1 (1983), in particular on page 84 thereof) it is reported that the micro convexo-concave structure of the organic thin film was directly produced by ablation of an organic macromolecule thin film, such as PMMA (polymethyl methacrylate) coated onto the glass substrate, using the interference light beam from an excimer laser.
In any one of the prior arts mentioned above, the thin film is formed on the surface of the substrate, thereby achieving a micro processing or machining thereon by absorbing the laser beam energy into the thin film to cause the ablation thereof. However, none of them takes into any consideration at all the energy of the laser beam.
Namely, although it was conventionally already known that a laser beam having an intensity higher than a certain level of energy must be irradiated to cause the ablation and so on, not only is the micro convexo-concave structure formed on the thin film, but also the substrate itself is processed or affected by the laser, in a case where the thin film is formed on the surface of the substrate. In particular, if the energy of the laser beam which reaches the substrate through the thin film is greater than a certain energy (threshold) that is enough to cause ablation on the substrate.
If micro processing on the substrate, which is different in physical property from the thin film, is being carried out at the same time, it cannot be used as an optical element, such as the diffraction grating, etc., for which is required a certain level of accuracy thereof.
Further, it has a disadvantage with respect to the characteristics of weather resistance and heat resistance, in the case where the thin film comprises organic macromolecules.
On the other hand, it is already known that a diffraction grating in which periodic convexo-concave structures are formed on a dielectric multiple film layer in one direction, as shown in FIG. 10(a), has superior characteristics as a polarized light beam splitter (Rong-Chung et al., OPTICS LETTERS Vol. 21, No. 10, p761, 1996).
Also, a diffraction grating in which periodic convexo-concave structures are formed on a dielectric multiple film layer in two directions, as shown in FIG. 10(b), has been proposed as a photonic crystal of three dimension (E. Yablonovitch, Journal of the Optical Society of America B Vol. 10, No. 2, p283, 1993).
At the present time, a dielectric multiple film layer itself has been widely used in various technical fields as a mirror, etc., and also various techniques have been already established as the method for manufacturing thereof, including an electron beam. evaporation method, a heating evaporation method and a sputtering method.
Also, since the technology for forming the periodic convexo-concave structures on a dielectric multiple film layer is similar to the so-called patterning technology for producing VLSI (very large scale integration), etc., a diffraction grating, in which periodic convexo-concave structures are formed on a dielectric multiple film layer, can be produced by adopting the patterning technology for producing VLSI on a dielectric multiple film layer.
More concretely, as the technology for such patterning, there is known a wet etching (chemical etching) method by using an etchant including hydrofluoric acid, etc., or a dry etching (physical etching) method by using a reactive ion etching, etc., which is applicable thereto.
It is possible to manufacture a diffraction grating and so on by adopting the above-mentioned film forming and etching method. However, as is mentioned above, there is a problem with the care and the treatment of the etchant in the wet etching, and also a problem in the dry etching in that the facility of a vacuum container or the like is necessitated, thereby requiring a large-scale facility by itself. Additionally, in the dry etching, it is not cost-effective because a pattern mask must be formed by a further complicated photolithography technique including the steps of resist film painting, drying, exposure, baking, development and so on.
Moreover, when processing the etching on the dielectric multiple film layer laminated with plural. kinds of layers, it is difficult to obtain a clear cross-section shape due to the difference in etching rates among the respective layers thereof.
Therefor, according to the present invention, for resolving such drawbacks mentioned above, there is provided a laser processing method to a glass substrate and an optical diffraction element obtained thereby with respect to the first and the second invention of the present application, comprising the steps of: forming a thin film of a material showing superior absorption characteristics of a laser beam to that of the glass substrate, on a surface of which said thin film is formed; radiating the laser beam having an intensity distribution onto said thin film; and removing the thin film depending on the intensity distribution of the laser beam by fusion, evaporation or ablation which occurs by making said thin film absorb energy from the laser beam radiated thereon, wherein said thin film is made of inorganic materials, and a thickness or an absorption index of said thin film with respect to an intensity of the laser beam is set at a value less than a threshold value that is enough for the laser beam to reach a surface of said glass substrate penetrating through said thin film to cause the fusion, evaporation or ablation.
As the thin film, there is appropriate a single layer or a plurality of layers of one or any combination of a glass of a metal oxide, a metal nitride, a metal carbide, a semiconductor, and silicon dioxide (SiO2), a fluoride glass, and a chalcogenide glass.
And as a method of forming the thin layer or film, various methods can be applied thereto, including a sol-gel method, a sputtering method, a vacuum deposition method, and liquid-phase epitaxy method, and so on.
Energy loss of the laser beam, which occurs when it passes through the thin film, can be controlled by controlling a thickness of the thin film and an absorption index. However, in a case where it is necessary to keep a certain thickness, it may be controlled by varying (mainly) the absorption index for the laser beam.
And as a method of controlling the absorption index, there are a method of intensively introducing a shift in a ratio of quantum theory, such as of defective oxygen, a method of introducing a defect, a method of doping an ion showing high absorption with respect to the wavelength used, a method of mixing amicrons or ultra fine particles, a method of mixing a pigment, a method of mixing an organic pigment, etc.
Further, it is possible to produce an optical diffraction element for use as a diffraction grating or a hologram installed in an optical coupler, a polariscope, a wave divider, a wavelength filter, a reflector, a mode transducer, etc., by using a laser beam having a periodic or regulative distribution in optical intensity.
Here, when forming the convexo-concave pattern on the thin film formed on the surface of the glass substrate by the laser beam, the thickness of the thin film becomes automatically to be equal to the thickness of the convexo-concave structure, by developing the ablation until the glass substrate appears at the bottom of the concave portion of the thin film.
A laser beam having such a periodic optical intensity distribution can be obtained by a phase mask, or by interference between the laser beams, and thereby the configuration of a cross-section of the periodic convexo-concave pattern formed on the surface of the glass substrate can be controlled by the pulse energy of the laser beam. Moreover, a laser beam having such a regulative optical intensity distribution can be obtained by using a net shaped mask, and so on.
On the other hand, a method for manufacturing an optical element with respect to the third invention of the present application comprises the steps of: forming a dielectric multiple film layer, which comprises plural kinds of layers having different permittivities on the surface of a substrate; irradiating a laser beam having an intensity distribution onto said dielectric multiple film layer; and removing a portion of the dielectric multiple film layer depending on the intensity distribution of the laser beam by fusion, evaporation or ablation which occurs by making said dielectric multiple film layer absorb energy from the laser beam radiated thereon, so as to leave the other portions as the periodically arranged dielectric convex portions having a grating constant corresponding to the wavelength of the beam.
Here, it is preferable that the material forming said dielectric multiple film layer has a threshold value to cause the fusion, evaporation or ablation with respect to the laser beam that is lower than that of the substrate, and that it has a large adhesion in the film thereof. Concretely, silicon oxide, titanium oxide, cerium oxide, germanium oxide, magnesium fluoride, calcium fluoride, tantalum oxide, etc., are appropriate.
Further, the film layer forming each layer of the multiple film layer can be formed as a glass (amorphous), in a single crystal or in a polycrystal.
In a case where said laser beam has the periodic intensity distribution in one direction, such laser beam can be obtained either through a phase mask or through an interference between two laser beams.
In a case where said laser beam has periodic intensity distributions in two directions, such laser beam can be obtained through an interference among at least three laser beams.
As a source of the laser beam, an excimer laser such as KrF, an Nd-YAG laser, a Ti:Al2O3 laser and the high harmonics thereof, or a pigment laser can be used, and preferably, a laser beam having a lower reflectivity with respect to the dielectric multiple film layer to be processed should be used.